It has long been known that colorless or pale-colored chromogenic substances, such as crystal violet lactone, and phenolic compounds can react to produce a color, and the use of such reaction in thermoresponsive paper sheet recording is disclosed in U.S. Pat. No. 3,539,375, for instance.
However, to meet the demands for higher thermal sensitivity and high-speed responsiveness, for instance, arising from recent advances in recording devices and diversified use of thermoresponsive recording sheets, it is still necessary to solve various problems. For instance, for use on thermal printers or thermal facsimile telegraphs, thermoresponsive paper sheets should have improved thermal responsiveness in color production, since an insufficient degree of responsiveness would result in increased electric power consumption and/or decreased printing velocity. For increasing color-producing responsiveness of thermoresponsive sheets, there has already been proposed the use of such additives as waxes (Japanese Patent Application laid open (Kokai) under No. 19,231/1973) and nitrogen-containing compounds (Japanese Kokai 34,842/1974).
In thermoresponsive recording sheets, presumably a chromogenic substance and a phenolic compound are present each in the stable and finely divided state dispersedly in the same layer or in different layers and, when heated, at least one of the two components melts or sublimates or both give an eutectic mixture, whereby they come into intimate contact with each other to produce a color. Therefore, it is necessary that each reactive color-developing component should be colorless or pale-colored crystals or solid at normal temperature, and further it is preferable that said component should melt at 70.degree. C. or over and completely liquefy and/or vaporize at 150.degree. to 200.degree. C.
U.S. Pat. No. 3,539,375 describes as a phenolic compound adequate for such purpose 4,4'-isopropylidenediphenol, which is used today in many cases.